Frequency modulator



INVENTOR 5 Sheets-Sheei l e. L. UssELMAN FREQUENCY MODULATOR Filed June 18, 1940 G. WM!

A'l Tt DRNEY Dec. 8 1942.

e. u USSELMAN FREQiIENCY MODULATOR Filed June 18, 1940 5 Sheets-Sheet? OUTPUT INVENTOR 63 'L. SELM N BY M} A'h'oRNEY I Patentedl Jec. a,

FREQUENCY MODULATOR- George L. Usselman, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation oi Delaware Application June 18, 1940, SerialNo. 341,119

g 8 Claims. The present application concerns frequency modulators comprising oscillators with detuned oscillation circuits entrained by coupling to opcrate at the same frequency and means for modulating the oscillators to frequency modulate the resultant oscillations. In the present invention modulation is accomplished by modulating the -elementpotentials of the oscillator tubes in the frequency modulator.

Indescribing my invention reference will be made to the attached drawings wherein:

Figs. 1 to 8, inclusive, each show a frequency modulator wherein two oscillation generators each including an electron discharge tube which would operate, when uninfluenced by the other,

i at diil'erent frequencies but which are entrained to operate at the same mean frequency. The

oscillators are then modulated so that the more the same manner it is applied to the grids in Figs. 1, 2 and 3.

Fig. 5 is similar'to Fig. 4 exceptthat Fig.- 5

shows a method. of applying the audio: signal oscillations to the anodes of the frequency-modulator. 7 Stage C of Fig. 5 shows the use of a twin screen grid tube VI in stage C instead of two eDarateftubes. Fig. 5 also shows how a frequency regulating means, F, may be used to regulate the average frequency of the modulator carrier frequency by use of such apparatus as that described in U. 8. Patents 2,044,749, 2,018,820

and 1,794,932. Means F operates condensers C3 and C4 to accomplish this purpose.

sr dsof thetubes are cross-connected and difiercontrol passes from one to the other and in so passing varies the frequency of the oscillations generated. In the modification shown in Figs. 9

and lO, a third oscillator tuned to the said mean frequency of operation is used in the system,

Figs. 1 to 8, inclusive, show various forms of o my invention. They all operate on' the same fundamental principles. The frequency modulator stage B consists of two Hartley oscillator circuits comprising tubes VI and V2. These two oscillator circuits tubes have their grids or their anodes cross-connected. Figs. 1, 3, 4, 5, 6, 'l and 8 show cross-connected grids in the frequency modulator. plied tothe tube grids, anodes or auxiliarygrids. Each side of the modulator is separately coupled at its output to stage C, not shown in Figs. 3 and 'I. The latter is coupled to stage D (not shown in Figs. 3, 4, 6, '7 and 8) which consists of frequency' multipliers or amplifiers, or both. These deliver their output to a load or antenna E. Fig.

2 is similar to Fig. 1, except that Fig. 2'shows the Signal oscillations are'apfrequency modulator using cross-connected ancuit B using pentodes or five element tubes instead of three element tubes. In Fig. 3 the control grids are cross-connected and'the number 8 grids or shielded grids act as anodes'ior the oscillator circuits. The anodes of the tubes Vi and V! are used for output coupling. In this circuit either the number I or number 8 grids may be cross-connected.

Fig. 4 shows the use of three element tubes VI and VIand neutralizing condensers in thei're quency. modulator circuit- In this circuit the grids are cross-connected and the audio signal oscillations are applied to the grids in somewhat odes; Fig. 3"shows the frequency modulator cirviouslv the modulating potentials In the moriflcation illustrated in Fig. 6, tubes of the screen grid type are used. The control ential anode modulation is used. In this modification as in all the prior modifications, it is important that individual oscillators be uncoupled or isolated except for the cross-connected electrodes. To do so, I provide shielding means SI, SI between the oscillators and circuits. In

all of the arrangements shown heretofore and hereinafter, grid or anode or screen grid modulation may be used. v

Fig. 7 illustrates a modification wherein each separate oscillator comprises two electron discharge devices having their anodes and grids connected in'push-pull oscillation generating circuits, the grids being cross-coupled to entrain the oscillators. In this modification differential anode modulation is accomplished although obpressed on other electrodes.

In the modification shown in Fig. 8, the oscillators are each of the crystal controlled screen grid type with the grids cross-connected and the anodes modulated differentially. In principle, all" of the modifications described hereinbefore,

are, in general, the same.

In the arrangementsshownin Figs. Qandiil, an additional electron discharge tube oscillator v1 is used. In this modification the three oscillators are each coupled to the other to form what has been termed a "ring oscillator system." In these modifications it can be said, in general, that the principle involvedin'the prior modifications is also involved. That is to say,

here the oscillators comprising tubes VI and VI are tuned to operate at frequencies on opposite sides of a mean frequency which is to be the carrier frequency in the absence of coupling between oscillators. The third oscillator comprising the tube V! and. its circuits is timed to operateat the may be imrespects.

carrier frequency. The oscillators are coupled and entrained so that as connected, coupled, and entrained they operate at a, single frequency. In the presence of modulation which is applied to the control grids of tubes VI and V2 in Fig. 9

and the anodes of the same tubes in Fig. 10, the oscillators cooperate to produce oscillations of a carrier or mean frequency equal to the frequency to which the oscillator including tube V! is tuned, and the frequency of th'ese oscillations swings in accordance with modulation.

The output may be taken from the circuits of tubes VI and V2 and impressed on separate electron discharge coupling systems as in Fig. 9 or the output may be taken from the circuit of tube V! and supplied to a single electron discharge" system as in Fig. 10.

Referring more in detail to Fig. 1 of the drawings, tube VI has its anode 8 connected with an :oscillatory circuit comprising condenser Ill and and frequency amplifiers, etc.

electron stream only to the anodes 8 and 8' and to the output circuit. In this modification, I have shown a resistance I0 tieing the plati 8 and 8' together. The output is derived from t tap on this resistance. This output may be su plied to an amplifier stage C comprising a single tube or two tubes and from the said stage to amplifiers The arrangement of Fig.4 is described hereinbefore in connection with-the prior figures and, in addition, I- have provided in the arrangement of Fig. 4 neutralizing condensers NI and N2 by means of which the separate electron discharge tubes VI and V2 and their circuits are neutralized to further uncouple the same and isolate the same for individual operation except for the coupling obtained by the cross-connections between the grid electrodes I8 and I8. The tubes by impressing modulating potentials from a" source A on the primary of transformer T and impressing the same from the secondary winding of T on. the control grids I8 and I8 in phase opposition. The leads between the transformer secondary and the grids I8 and l8f include resistors 20 and 24.

VI and V2 here, as in the prior modification, are shieldedeach from the other and from the following-stages by shields SI and S2.

p In the arrangement of Fig.5, I have replaced the stage C by a single electron discharge device V3 having two electrode systems one of which is coupled to the oscillation circuit comprising con-,

denser I0 and inductance I2 and the other of which is coupled to the oscillation circuit comprising condenser III' and inductance I2. In this modification I have provided means in the form of variable condensers'flfl and 80' connected with a frequency regulating system 84 the function of which is described more in detail hereinafter. I I I In the modification shown in Fig. 6, the control grids of the oscillator are cross-connected.

' Modulation is. applied differentially to the anodes The frequency modulated oscillations are supplied from each oscillator by way of leads 3B and 32 to the control electrodes 36 and 314' of coupling tubes V3 and V4. The anodes 36 and 36 of these coupling tubesare coupled together and connected to a parallel tuned circuit 40 which may supply the frequency modulated wave energy to an amplifier and/or frequency multiplier 46.

The circuit arrangement shown' includes radiofrequency'choking coils so designated, blocking condensers designated by the letters BC and bypass condensers designated by the letters BP. The function of these elements is known to those of the tubes and the tubes are of the screen grid type. y.

In the modification shown in Fig.7, each of the oscillators comprises two electron discharge devices VI and V5 and V2 and V6, each pair of deviceshaving their anodes 8 and BI and 8' and BI connected in push-pull by the oscillation generating circuits comprising inductance I2 and condenser l0 and inductance I2 and condenser Ill. The grids I8 and 83 are connected to points on the inductance I 2' while the grids I8 and 83 are connected to points on inductance I 2. Moduskilled in the art and it is believed unnecessary to describe the same in detail herein.

The modification shown in Fig. 'Zdifiers from the modification shown in Fig. l in the following nected instead of the grid electrodes I8 and I8 and the biassing resistors 20 and 24 and asso- 'ciated bypass condensers BP have been rearranged because the gridsin this arrangement are not cross-connected as in Fig. 1. In this modification variable condensers and Ell are connected between the control grids and the 'thethird grid 64, shielded by the second and fourth grids 60 and 62 connected together, serves as the anode of the oscillation generating circuits which, in this modification, are coupled by the The anodes 8 and 8' are cross-conlation is applied on the anode electrodes 8 and 8!. The remaining elements such as resistances, blocking condensers, radio chokes and bypass condensers have been designated by reference symbols used in the prior figures and need no further description.

In the modification shown in Fig. 8, the oscillators are of the piezo-electric'crystal type. The tubes V8 and V2 have their anodes 8 and 8'- and control grids l8 and I8 connected in oscillation generating circuits including crystals XI. and X2, respectively. That is, the, crystal X2 is coupled between the grid I8 of tube VI and the anode 8' of tube V2. The crystal XI is coupled between the grid I8 of tube .V2 and the anode 8 of tube VI. .The anode I8-of tube VI is coupled by a blocking condenser BC and radio-frequency choke to the control grid 34 of coupling and amplifying tubeVZI. A similar coupling is provided between the anode'8" of 'tube VI and the control grid 34 of tube V2.

The piezo electric crystal holders are variable topermit the frequency of the oscillations gen erated in the respective oscillation generator circuits to be varied the amount necessary to cause one oscillator, say tube VI 'and crystal XI, to operate above and the other oscillator, say tube .ation of this frequency modulator.

v r 2,804,888 s g "V2 and crystal xi, to operate below the mean frequency at which they operate when entrained by cross-coupling the control grids l8 and I8. Plate modulation in a differential manner is used in this modification by coupling the anodes 8 and l to the secondary windingof transformer T.

In the modification shown in Figs. 9 and 10, I

- have supplied an additional electron discharge device V'Iof the screen-grid type ha ing an anode Hill connected in Fig.- 9 to a tuned circuit comprising'inductance H8 and condenser H4. A

point on inductance H8 is coupledby condenser BC to grid ll of tube VI. The grid H of tube V1 is coupled by a condenser BC to a point on inductance l2 and the grid it of tube V2 is coupled by a condenser BC to a point in the inductance l2.

In Fig. 10 the'anode Hill of tube V1 is connected to a crystal X! which is coupled to ground by a bypass condenser BP andto the grid I8 oftube VI. The grid electrode I of tube V1 is connected or coupled by a coupling condenser BC to a point on the inductance l2 of tube V2.

The control grid ll of tube V2 is coupled by a condenser BC to a point on the inductance l2 in the anode circuitoftubeVl.

In Fig. 9 modulation is accomplished bymcdution of Fig. 10 modulation is accomplished by modulating, the potentials of anodes 8 and 8 of tubes VI and V2 by coupling the same to the secondary winding of transformer T.

similar except that in Fig. 9 modulation is applied differentially to the control grids l8 and it through the secondary winding of transformer T. Moreover, in the modification shown in Fig. 9, separate coupling-tubes V3 and V4 are used in stage C as in some of the prior modifications whereas inFig. 10 the. output is derived from a point on the circuit of tube V! and is supplied to a single tube V8 in the stage C.

. lating the potentials of the grids l8 and I8 of tubes Vi and V2 differentially. Inthe modifica- Figs. 4 and 5 will be used todescribe the oper- It. will be noted that each modulator consists of two-oscil-. lator circuits. These circuits are shielded from each other by shields SI and S2 so that the only coupling they have to each other is through the cross-connected grids or the cross-connected anodes as the case may be. The input and output circuits are carefully arranged to cause no coupling between the two sides of the modulator.

In starting up the transmitter, proper steady cathode, grid and anode voltages are applied to ative amount of power supplied by each of the modulator tubes VI and V2 and by the frequencies to which the modulator tank circuits are frequency.

In operation, both sides .(both halves) of the modulator oscillate approximately in phase.

Thatis, the anode voltages of tubes VI and V2 rise and fall about 'in phase and the grid voltages rise and fall about in phase but there is some phase difference between the two oscillators. However, if one tube, say VI, is biased to deliver morepower than tube V2 then the output frequency will shift toward that for which the tank circuit I0 and I2 of tube Vl' is tuned. If tubes Vi and V2 are biased so that tube V2 delivers more power than tube VI, then the output fre-- quency will shift toward the frequency for which the tank circuit l0 and I2 of tube V2 is tuned.

-'I"he amount of frequency shift and direction will depend on the direction and amount of bias and tank circuit tunings.

Now, it can be seen that by differentially changing the direct-current potentials of either the grids or the anodes with proper means, the carrier frequency of the transmitter" can be frequency modulated in accordance with the signal oscillations. In other words, the signal oscillations will cause the modulator output frequency to shift above and below the average, or carrier, frequency in accordance with the signal oscillations. The amount of frequency shift will be proportional to the signalamplitude and the frequency of the shift will be the signal frequency. This frequency modulator is one. in which the usual amplitude modulation is balanced out; therefore, no distortion is caused from this source.

As stated previously, Figs. 1 to .4, inclusive,

illustrate a method of operating the modulator by varying the grid potentials according to the signal oscillations from unit A through trans former T. Fig. 5 illustrates a method of operating the frequency modulator by varying the anodepotentials according to the. signal oscillations from. unit A through transformer T.

The output from each half of the modulator is coupled to .stage C in such a way that the two energies add but so that there is no coupling at this point between the two sides of the modulator. -It is usual to amplify and multiply the frequency of the modulatoroutput before it is radiated in antenna E.

It may be noted that in Figs. 4 and 5 neutralizing condensers NI and N2 are used to neutralize capacity feedback on the same side of the circuit and that the grids are cross-connected to tune the other half of the modulator sub- .stantially the same amount above the carrier frequency. This adjustment is made by means of tuning condensers I0 and 10'. The amount of adjustment depends on the desired frequency neously. That frequency is determined by the relthrough direct-current blocking condensers B03 and BC! to the opposite tank circuit. The same purpose is accomplished in Fig. 3 by the use of multi-grid tubes.

the modulator depends on the opposite half for This means that each half of mitter to a required value. Unit'F may be apparatus as described in my U. S. Patents 2,044,

749, 2,018,829 and 1,794,932. To make the latv ter apparatus operate as desired, part of the modulator or transmitter energy is coupled to the detector input, as disclosed in Patent 1,794,932,

and the automatic tuning gear, suchas disclosed in Patent 1,794,932, or frequency correcting parts (20, 22, 24, 26, 42, etc.) are'connected to auxiliary condensers C3 and C4 of the present invention. This would cause the apparatus to' act as a frequency regulator in the present in-" vention and it would tend to hold the transmitter frequency on line Fm and in band C-D of Fig. 2, Patent 1,794,932.

In Fig. 6, I have shown the frequency modulator and stage C using screen grid tubes VI, V2, V3 and V4. This circuit illustrates the use of anode modulation. In fact both anode and grid modulation were tried at different times on this circuit. The screen grids for each tube of v the frequency modulator may or may not be modoscillator has the tube connected across half of the tank circuit. This is equivalent to having a variable impedance connected across part of one branch of a parallel tuned circuit so that there is a tendency to change the resonant frequency of th tank circuit as the tube impedance is varied. Thistendency is not appreciable but it is largerin the screen grid circuit, such as Fig. 6, than it is in the neutralized circuits. to overcome this detuning effect mentioned above, I have shown in Fig. 7, a frequency modulator in which push-pull oscillators are used. Fig. '7 illustrates the use of screen grid tubes in this circuit but three element tubes may be used by providing neutralizing condensers. This push-pull modulator overcomes the slight frequency modulation distortion present in Figs. 1 to 6, inclusive, due to each oscillator having a variable tube impedance connected across only one half of each tank circuit. Fig. 7 has a tube impedance connected across both halves of each gtank circuit. These tubes in each oscillator have like variations in impedance as the modulation varies each oscillator potential differentially. In Fig. 7 no such frequency modulation distortion takes place; The circuit in Fig. 7 operates on the samegeneral principles as the preceding circuits. The cross-connected oscillators of the modulator should be well shielded from each other, having only their grids or their anodes cross-connected. The frequency modulator may be considered as two cross-connected oscillators or as two cross-connected amplifiers, each amplifying oscillation energy from the other, in which their anode tank circuits are tuned differently or off frequency from each other. In order to get afrequency' modulated carrier signal: it is only necessary todifierentially modulate the amplitudes of the oscillations in each oscillator. The amplitude of the sum of the energy output will be substantially constant but the frequency will be modulated."

The principle of cross-connected oscillators. to compose this frequency modulator may be carried over into the use of quartz crystal oscillators,

resonant line oscillators, and to theuse of 'Haefi tube oscillators. Fig. 8 illustrates a frequency modulator using In order two cross-connected crystal oscillators. These crystals have their resonant curves partly over lapping. The crystal holder air gaps are made adjustable to get the desired frequency sweep for the frequency modulator. Fig. 8 shows the modulating potentials being applied to the anodes,

but they could just as well be applied to the grids. Other crystal oscillator circuits may be used in this type of frequency modulator circuit.

Figs. 1 to 8, inclusive, show frequency modulators consisting .of two cross-connected oscillators. It is possible to cross-connect or rather series-connect-any number of oscillators .or amplifiers in a closed ring.

-In Fig. 9 I illustrate a somewhat different type of frequency modulator where this principle is carried out by using three oscillators (or amplifiers). Each oscillator is connected to and excited by the suceeding' oscillator so that they are connected in a closed ring. All three oscillators oscillate on one frequency simultaneously. In this circuit two of the oscillators BI and B3 are modulated while oscillator B2 is not moduf lated. In normal operation as a frequency modulator, one oscillator, say B2, is set at the desired fixed frequency or carrier frequency. This oscil- 1iator,B2, may be a coaxial line'type of oscillator, a crystal controlled oscillator or any other desirable type of oscillator for holding an average frequency. Then the oscillator BI is tuned say for a frequency slightly below that of oscillator B2 and oscillator B3 is tuned the same amount, say for a frequency slightly above that of oscillator 132. Now, according to tests, the frequency will be pulled toward that of the oscillator having the most circulating energy (current) in its tank circuit. Consequently,.if oscillators BJ and B3 be differentially modulated so that alternately they have more oscillating current and deliver more controlling voltage than oscillator B2, then the frequency of oscillator B2 and the entire circuit will be shifted alternately to one side and to the other favoring the oscillator having the most power at. that instant. When no modulation is present, oscillators BI and B3 have equal power. Then oscillator B2,which has more controlling energy in its tank circuit and being of lower power factor (lower loss), will control the frequency. If oscillators Bi and B3 are differentially modulated'according to the amplitude and frequency of the signal, then the modulator output will be frequency modulated according to the signal. The advantage of the modulator shown in Fig. 9 is that a better average or a.

p more steady carrier frequency is produced. The

' I Fig. 10 illustrates a frequency modulator having three'oscillators in which the unmodulated oscillator B2 is a crystal controlled oscillator.

It may be noted that in Fig. 9.1 show grid modulation on oscillators BI and B3 and the outputs of oscillators BI and B3 are both combined in stage C. In Fig. 10, I show anode modulation on oscillators BI and B3, while the out-. put of osclllatorB2 only is delivered to stage C. The circuits in both Figs. 9 and 10 will give a frequency modulated current of substantially constant amplitude. As stated before, all os-' cillators and stage C should be well shielded from each other. These figures show the grids of each of the oscillator tubes being excited from the succeeding stage. This order can be reversed so that the grid of each of the oscillator tubes is connected to and excited by the p stage.

It should be stated that the two oscillators which are modulated in Figs. 1 to 10, inclusive, should have substantially the same L-C ratio and should be identical otherwise to give linear frefl quency response. Oscillator B2 in Figs. 9 and 10 is the exception. .What is claimed is: e

1. In a frequency modulation system, at least, threeioscillation generators each comprising a tube and a circuit, tuned to a diflerent frequency coupled to the tube electrodes, connections coupling corresponding electrodes of each tube to a point on the tuned circuit of another tube to connect said tubes and circuits in a ring connectedoscillator operating substantially at asingle frequency, and a circuit for modulating the impedances of two of said tubes differentially v in accordance with signals.

2. In a frequency modulation system, a pair of tubes eachhaving an anode, a cathode and a grid, a circuittuned to a'first frequency coupling frequencies coupling the anode of said third tube to the cathode of said third tube, a coupling between an electrode in each of said tubes and a' tuned circuit coupled with the electrodes of another of said tubes, connections for energizing the electrodes of saidtubes whereby oscillations are produced in each of said tubes: and conneccoupling between the control grid of said one tube apoint on said first circuit on the other side of and a point on said second circuit at the other side of said intermediate point thereon, a coupling between the control grid of said other tube and the intermediate point thereon, a third tube having electrodes coupled to a circuit tuned to a third frequency in oneof said last two named couplings, and means for modulating two of said tubes in push-pull relation at signal frequency.

5. In a wave length modulation system, a pair of electron discharge tubes. each having an anode,

circuits, means coupling the-control grid of said other of said tubes tosaid one of said tuned.

tions for modulating the impedances of two of said tubes in accordance with signals to thereby modulate the frequency of the oscillations produced by said tubes.

3. In a frequency modulation system, a pair of tubes each having. an anode, a cathode and a grid, a circuit tuned to a first frequency coupling the anode and cathode of one of said tubes,

a circuit tuned to a second frequency cou pling the anode and cathode of. the other of said tubes, a third electron discharge tube having an anode, a cathode and a grid, a piezo-electric crystal coupling, the anode of said third tube to the grid of one of said pair of tubes, a coupling between the grid of the other of said pair of tubes and the circuit coupling the anode and cathode of said one .oi-said pair of tubes, a coupling between the grid of said third tube and the circuit coupling the anode and cathode of said other of said pair of tubes, connections for energizing the electrodes of said tubes whereby oscillations are produced in said tubes and circuits, and connections for modulating the impedances of said pair of tubes differentially in accordance with signals to thereby modulate the frequency of the oscillations produced.

4.- In a wave length modulation system, a pair of electrondischarge tubes each having an electrodeserving as an anode, a cathode and a control grid, a first circuit tuned to a first frequency,

said first circuit having two terminals, a second circuit tuned to a second frequency, said second theother of saidtubes, a couplingbetweenthe 75 a cathode and a control grid, a first circuit tuned to a first frequency, a second circuit tuned to a second frequency, means coupling the anode of one of said'tubes to one of said tune'd circuits, means coupling the anode ofthe other of said tubes to the other of said tuned circuits, means coupling the cathode of said one of said tubes to a point on said one of said tuned circuits, means coupling the cathode of the other of said tubes to a point on said other of said tuned circuits, means coupling the control grid of said one of said tubes to said other of said tuned circuits, a third electron discharge tube having electrodes coupled in a circuit normally operative at a third frequency in one of said last two 5 coupling means, and means for modulating the impedances of two of said tubes differentially at signal frequency.

i 6. In a wave lengthmodulation system; a" pair of electron discharge tubes each having an anode, a cathode and a control grid, a first circuit coinprising an inductance and a capacity in parallel tuned to a first frequency, a second circuit comprising an inductance and capacity in parallel tuned to a second frequency, connections coupling the anode of one of said tubes to the ter-' minal of one of said circuits,'connections coupling the anode of the otherof said tubes to the terminal of the other of said circuits, connections coupling the cathode of said one of said tubes to a point on said one of said circuits, connections coupling the cathode of said other of said tubes to a point on said other of said circuits, connections coupling the control grid of said one of said tubes to a point on said second circuit, connections coupling the control grid of the other of said tubes to a point on said. first circuit, and circuits for modulating the impedances of said tubesdiiferentially in accordance with signals.

of electron discharge devices each having .an

anode, a cathode and a control grid, a frequency determining reactance operating, at a first frequency, a second frequency determining reactance operative at a second frequency, means coupling theanode of one of said tubes to one end of said first reactance, means coupling the anode of the other'of said tubes to one end of said second reactance, means coupling the cathode of one of said tubes to a point on said first reactance and to a point of low radio frequency potential means coupling the cathode of the other of said tubes to a point on said second reactance and to a point of low radio frequency potential, means coupling the control grid of said one of said tubes to the other end of said second reactance, means coupling the-control grid of the other of said tubes to the other-end of said first reactance, means for modulating the impedances oLsaid tubes differentially at signal frequency, a pair of amplifier systems each having a control grid and an anode, means coupling the control grids of saidsystems to said reactances, and means coupling the anodes of said tubes in parallel.

8. In a modulation system, apair of electron discharge tubes each having an anode, a cathode and a control grid, a first circuit including an inductance tuned to a first frequency, a second circuit including an inductance tuned to a second frequency, means coupling the cathode of one of said tubes to a point intermediate the terminals of the inductance of said first circuit,

" means -coupling the cathode of the other of saidtubesto a point intermediate the terminals of 1 the inductance of said second circuit, means coupling the anode of said one tube to one end of the inductance of the first circuit, means coupling the anode of said other tube to one end of the inductance of the second circuit, means coupling the control grid of said one tube to a' "amplifying the modulated oscillations comprising a pair of discharge systems having input elec-' trodes tapped to said inductances and having output electrodes coupled in parallel.

GEORGE L. USSELMAN. 

